Matt Edmondson, a hacker specializing in digital forensics, built an anti-tracking tool with a Raspberry Pi that "scans for nearby devices and notifies you if the same phone is tracked multiple times in the last 20 minutes." he says the Wired.
The device, which can be carried or mounted in a car, consists of parts that cost about $200 in total. 
The system works by scanning for wireless devices around it and then checks its logs to see if it was around in the last 20 minutes. Designed to be used while people are on the move. The anti-tracking tool can be placed inside a case about the size of a shoebox, and consists of a few components.
A Raspberry Pi 3 runs its software, and a Wi-Fi card searches for nearby devices. A small waterproof case protects it and a portable chargers feeds the system.
A touch screen displays the notifications the device generates. Every notification can be a sign that you are being watched. The device runs Kismet, which is a wireless network detector and is able to detect smartphones and tablets around it looking for Wi-Fi or Bluetooth connections. The phones we use are constantly searching for wireless networks around them, including networks they've connected to in the past.
Edmondson says the code is written in Python to generate lists of what Kismet detects over time. There are lists for devices detected in the last five to 10 minutes, 10 to 15 minutes, and 15 to 20 minutes. If a device appears twice, a notification will flash on the screen. The system can display a phone's MAC address, although this isn't very helpful if it's been hacked.
It can also record the names of Wi-Fi networks that devices around it are searching for.
To stop the system from detecting your phone or other people traveling with you, it has a list that allows you to remove as many devices as you want.
Edmondson says that in the future, the device could be modified to send a text notification instead of displaying them on the screen. He's also interested in adding the ability for tire pressure monitoring systems to detect repeated nearby vehicles. One could also be added unit GPS so you can see where you were when you were being tracked.
Edmondson doesn't want to make the device a commercial product, but says his project could easily be copied and reused by anyone with some technical knowledge.
For those interested, Edmondson released his open source and plans to present the project at the conference security Black Hat in Las Vegas this week.
